This invention relates to an optical pickup device for condensing the light from a light source on a signal recording surface by an objective lens and for detecting the return light from the signal recording surface by light detection means.
For recording/reproducing optical or magneto-optical signals on or from an optical recording medium, such as an optical disc or a magneto-optical disc, an optical pickup device, which radiates a laser light beam to the optical recording medium and detects a return light beam therefrom, is employed.
The general construction of the optical pickup device is shown for example in JP Patent Kokai Publication JP-A-54-39101 (1979), shown herein in FIG. 1. This conventional optical pickup device is so constructed that a laser light beam from a laser diode 71 is condensed by an objective lens 71  73 on a signal recording surface of an optical disc 74 and a return light therefrom is detected by a photodetector 75 to produce a servo signal and an RF signal. Thus the optical pickup device is of a simplified design of a finite multiplication factor which has omitted the collimator lens. In particular, the optical pickup device includes a beam splitter 72 disposed obliquely between the objective lens 73 and the photodetector 75.
With the present optical pickup device, part of the laser light radiated from the laser diode 71 is reflected by the surface of the beam splitter 72, obliquely arranged between the objective lens 73 and the photodetector 75, so as to be directed to the objective lens 73. The objective lens 73 condenses the laser light to radiate it on the signal recording surface of the optical disc 74.
The return light reflected by the signal recording surface on the optical disc 74 again reaches the beam splitter 72 via the objective lens 73. The beam splitter 72 permits part of the return light to be passed therethrough to fall on the photodetector 75.
With recent increase in the volume of information, the optical recording medium, as the package medium of the music or picture information, such as a recording device for a computer, compact disc or a video disc, have become promulgated extensively, while the tendency is also towards recording with higher recording density.
Among the methods for achieving such high density recording, it may also be contemplated to increase the numerical aperture (NA) of the objective lens so as to be larger than that used in an optical pickup device employed in, e.g., a conventional compact disc player. If NA is increased, the beam spot formed on the compact disc is reduced in size, thus leading to improved resolution and higher recording density. However, if NA is increased, the tolerance for disc tilt is diminished.
For example, the optical disc reproducing device reproduces signals recorded on the reflective surface, that is the signal recording surface, via a transparent substrate having a thickness on the order of 1.2 mm. Thus, should the disc be tilted with respect to the optical axis of the objective lens, the third-order coma aberration, generated in proportion to approximately a third power of the numerical aperture NA and approximately a first power of a skew xcex8, becomes dominant. The generated wavefront surface W is given by
W(x, y)=W31x(x2+y2)xe2x80x83xe2x80x83(1)
On the other hand, the Seidel""s third-order coma aberration coefficient W31 becomes                               W          31                =                                                            n                2                            -              1                                      2              ⁢                              n                2                                              ⁢                                    tθNA              3                        /            λ                                              (        2        )            
because xcex8 is sufficiently small. The unit is standardized by the wavelength xcex.
In the above equations, t denotes a thickness of the disc substrate, n denotes a refractive index of the disc substrate, xcex8 denotes the quantity of disc skew, and NA is the numerical aperture NA of the objective lens.
In a system having the numerical aperture NA of, e.g., 0.6, which is as much as 1.33 times the numerical aperture NA of 0.45 of an objective lens employed in an optical pickup device of a conventional optical pickup device, a coma aberration of about 3.5 times as much as that produced with a conventional system is produced for the disc skew which is of the same magnitude as that of the conventional compact disc.
For example, if a disc having a disc skew as large as xc2x10.5 to 1xc2x0, such as a disc having a polycarbonate substrate produced inexpensively in large quantities, is employed, the spots  spot formed on the disc becomes non-symmetrical due to such wavefront distortion, thus increasing inter-symbol interference, such that waveform distortion in the reproduced signal becomes significant and hence the signal cannot be extracted sufficiently.
Thus it may be contemplated to reduce the thickness t of the disc substrate from, e.g., 1.2 mm to, e.g., 0.6 mm, that is to a one-half value, for reducing the third-order coma aberration coefficient W31 to a one-half value.
Meanwhile, if the disc substrate is of a small thickness, as described above, and a pre-existing optical recording medium, such as a write-once optical disc, phase-change optical disc or a magneto-optical disc, is reproduced using an optimized objective lens, RF signals of excellent signal quality cannot be reproduced due to the difference in substrate thicknesses.
It is known in general that the amount of fourth-order spherical aberration, standardized with the wavelength generated with parallel flat plates in the converging light beam, is proportional to the fourth power of the numerical aperture NA of the objective lens and proportional to a reciprocal of the wavelength. On the other hand, the wavefront W is given by
W(x, y)=W40(x2+y2)2xe2x80x83xe2x80x83(3)
The Seidel""s fourth-order spherical aberration coefficient W40 is given by                               W          40                =                                                            n                2                            -              1                                      n              3                                ⁢                      t            8                    ⁢                                    NA              4                        /            λ                                              (        4        )            
where t and n denote the thickness and the refractive index of the substrate, respectively.
Thus, if a compact disc having a substrate thickness t=1.2 mm is reproduced using an objective lens of a numerical aperture of 0.6, optimized for the substrate thickness t=0.6 mm, the amount of the generated spherical aberration reaches a larger value of 3.6 xcexcm in terms of the Seidel""s fourth-order aberration coefficient W40. The root mean square sum of the aberration of the optical system in this case is 0.268 rmsxcexcm. For comparison, root mean square sum values for the wavelength xcex of 0.532 xcexcm and for the wavelength xcex of 0.68 xcexcm are 0.5 rmsxcex and 0.4 rmsxcex, respectively.
It is generally required of the optical disc that the root mean square sum of the aberration of a given optical system be not more than 0.07 rmsxcex in terms of the Marechal criterion. Thus, such a system in which the root mean square sum ascribable only to the disc substrate amounts to as much as 0.4 rmsxcex to 0.5 rmsxcex is not acceptable.
That is, if two or more sorts of the optical disc having different disc substrate thicknesses are to be reproduced, playback signals of high signal quality cannot be obtained with a disc having a substrate thickness for which the objective lens is not optimized.
It is therefore an object of the present invention to provide an optical pickup device whereby high-quality recording/reproduction may be achieved even with two or more different optical recording media.
With the optical pickup device according to the present invention, the size of the aperture of the objective lens for condensing the light from a light source is varied depending upon the sorts of the optical discs in order to enable recording and/or reproduction of at least two different sorts of optical recording media.
With the above optical recording medium, it is possible for the aperture varying means to set a numerical aperture NA of the objective lens with respect to the optical recording medium having a substrate thickness of 0.6 mm to 0.6 as well as to set a numerical aperture NAo of the objective lens with respect to the optical recording medium having a substrate thickness of 1.2 mm to not more than (0.45/0.78)xc3x97xcexo, where xcexo denotes a numerical value equal to the wavelength of light radiated by the light source.
The aperture varying means for varying the size of the aperture of the objective lens depending upon the sorts of the optical recording medium may be arranged between the objective lens and the light source or between the objective lens and the optical recording medium. Alternatively, the aperture varying means may be arranged on the surface of  or within the inside of the objective lens.
The aperture varying means may also vary the aperture of the objective lens depending upon the substrate thickness of at least two different sorts of the optical recording media. The aperture varying means may also vary the aperture size by rotating respective blades of a light stop mechanism having the plural blades.
The aperture varying means may also vary the aperture size using a liquid crystal shutter or by moving of  or rotating a plate having plural openings for changing over the openings.
With the optical pickup device according to the present invention, since the size of the aperture of the objective lens for condensing the light from a light source is varied depending upon the difference in substrate thickness of the different sorts of the optical recording media by aperture varying means, high-quality recording/reproduction may be realized for disc-shaped recording media of two or more different sorts.